This invention relates to electronic signal compressors and, more particularly, to an audio compressor for use in an amplitude modulated transmitter.
Compression amplifiers are well known in the art. Such amplifiers are commonly used in audio signal applications wherein the dynamic range of an audio signal must be compressed to adapt the signal to a particular function. For example, audio compressors are used in the recording of music on tape and on phonograph disc. In particular, audio compressors are utilized in radio frequency signal transmitters to maintain a given index of modulation in the RF signal. Thus, amplitude modulated radio frequency transmitters are known to employ audio compressors as an attempt to maintain the percentage modulation within a given maximum.
A problem suffered with prior art amplitude modulated transmitters which employ audio compressors is that the modulation index is a function of DC bias to the radio frequency amplification stage. Generally, as the DC bias to a class C RF amplifier decreases, a given audio signal will modulate the RF amplifier to a higher modulation index. Conversely, as the DC bias voltage increases, percent modulation decreases. This problem is particularly acute in mobile operated transmitters wherein the DC bias voltage may undergo a substantial change in value.